1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission.
2. Description of the Prior Art
Generally, an automatic transmission for an automotive vehicle is provided with a torque converter and a shift gear mechanism of which a plurality of frictional elements, such as a clutch, brake are selectively actuated to switch a power transmitting path to thereby establish a desired shift stage automatically.
The automatic transmission includes a hydraulic control unit for controlling a hydraulic fluid to and from an actuator of the frictional element. The hydraulic control unit is provided with a regulator valve for adjusting a hydraulic pressure from an oil pump to a line pressure, a manual valve for manually switching a shift range, a plurality of shift valves for selectively actuating the frictional elements, and auxiliary valves actuated in connection with various operations. In recent years, the shift valve has been controlled through a solenoid so as to accomplish a sophisticated shift control in response to a vehicle operating condition.
In such kind of the automatic transmission, as disclosed in Japanese Patent public disclosure (JP A2) No. 63-186055, a low reverse hydraulic circuit for supplying a hydraulic pressure to a low reverse brake is provided independently from a 3-4 clutch hydraulic circuit for supplying a hydraulic pressure to a 3-4 clutch and operates independently from each other so as to prevent a double lock situation in which both the low reverse brake and the 3-4 clutch are engaged concurrently.
However, In such an automatic transmission as aforementioned, it is impossible to engage the 3-4 clutch when a L(low)-range or 1-range is selected in the manual valve that a higher shift stage, for example a third stage cannot be established in the L-range.